SQL Question

1. Write a simple subquery and display the course_no, description, cost for the course with the lowest course cost.

This is what I have so far but it is not correct what am I doing wrong?
SELECT course_no, description, cost
FROM COURSE
WHERE cost < ANY
(SELECT cost
FROM COURSE);

I have no idea how to even start answering question #2 and #3.
2. Write a correlated subquery and display the student_id, last_name,first_name of students enrolled in three or more classes.
a. Use the EXISTS or NOT EXISTs operator Hint: The HAVING clause can be used as well
b. Use the IN operator in a subquery

3. Write a correlated subquery and display the instructor_id, first_name, last_name, zip columns from the instructor table where the zip code is NULL.
a. Use the NOT EXISTs operator
b. Use the NOT IN operator